As part of a now concluded measurement campaign in the GW-LCYCLE project, EOC scientists were able for the first time to record from an aircraft atmospheric gravity waves at an altitude of some 87 kilometres. The FAIM-02 infrared camera on board DLR’s “Falcon” research aircraft was able to record at high horizontal and temporal resolution the radiation emitted from rotationally/vibrationally excited hydroxyl (OH) in the atmosphere’s so-called airglow and to make visible the influence of gravity waves there. Ground measurements with two infrared spectrometers (GRIPS-9 and GRIPS-14) and the infrared camera FAIM-03 were simultaneously made at Kiruna (Sweden) and Andenes/ALOMAR (Norway).
The dynamics of the atmosphere are dominated at different scales by waves. Since they can transport energy long distances it is essential to know where they arise, in which direction they propagate, and where they release their energy to the environment. Especially the last two questions have not yet been adequately investigated.
In the GW-LCYCLE project, investigating the life cycle of so-called gravity waves is of particular interest. Their name of these waves comes from the gravity that compensates for vertical deflection, for example of air flowing over a mountain. This gravity-induced restoration on the one hand and the inertia of the object deflected on the other hand result in a flow of wave motion.
The influence of gravity waves on the atmosphere can be directly detected by the temperature distribution in the mesopause. The mesopause is located at an altitude of about 87 km. Intuitively, one would expect at this altitude colder temperatures over the North Pole in winter than over the North Pole in summer, since the latter, along with the earth’s surface, is heated throughout the day by permanent solar irradiance. But in fact the exact opposite temperature distribution is observed: at the mesopause polar summer is colder than polar winter (see Figure 1). This effect is a consequence of breaking gravity waves and their influence on the large-scale circulation of air in the atmosphere. It appears that dynamic processes can significantly disrupt conditions of radiative equilibrium in the atmosphere at this altitude.
In order to take measurements in the mesopause, DFD monitors the so-called OH* airglow, which is a phenomenon caused by specific chemical reactions between hydrogen and ozone taking place at this altitude. Gravity waves modify the airglow; the changes are indicated by brightness and temperature oscillations.
Measuring in this layer from the ground to make gravity waves “visible” is hindered by clouds. For that reason, measuring from the top of high mountains, for example, yields better results on principle. The ideal case is to measure above the clouds from on board an aircraft. Using a customized camera installed on the FALCON research aircraft, undisturbed recordings could be made during particularly interesting weather situations as part of a measurement campaign lasting some four weeks in northern Sweden from mid-January to mid-February.
Figure 2 shows an image of OH* airglow at 87 km altitude (left) with the associated difference image (right). The latter reveals the discrepancies between two images, making also slight differences more clearly visible. The images show a wave structure with a horizontal wavelength of ca. 30 km in a north-south direction. The difference image also brings to light additional small-scale structures in the range of 3 km to 6 km. The flight path is indicated in the centre illustration. The colour range from blue to orange gives the average measured intensity at the respective coordinate, which makes it possible to analyse large-scale gravity waves. The location of additional ground-based instruments in Kiruna and ALOMAR is marked by a large and a small square. Gravity waves could be successfully monitored during all six flights.
These basic data were supplemented with images from a similar camera system on the ground equipped with a fish-eye lens that can capture the entire night sky (see Figure 3; the field of view is indicated by the circle on the map in Figure 2) and with two GRIPS (GRound based P-branch Infrared Spectrometer) instruments located in Kiruna, northern Sweden and ALOMAR, northern Norway (see the map in Figure 2). The infrared spectrometers contribute the temperature variations in the mesopause at the corresponding stations.
This highly successful measurement campaign came to an end with the dismantling of the last ground-based instrument at ALOMAR. The measurements make a valuable contribution to the global NDMC network (Network for the Detection of Mesospheric Change.
The GW-LCYCLE project is sponsored by the German Federal Ministry of Education and Research. The aerial measurements are being carried out as part of the POLSTRACC/GW-LCYCLE/SALSA measurement campaign, which is coordinated by KIT (Karlsruhe Institute of Technology), DLR and Frankfurt University. The overall project management for GW-LCYCLE is the responsibility of Prof. Markus Rapp, DLR-IPA.